SQL query for filtering by custom field

here is the site I’m working on:
http://speedways.postmodernsublime.com/nsd/tracks/
It’s a directory of race tracks. In the top right corner, next to the map, I want to be able to filter the listings by state. So I’m entering the tracks’ states using a custom field, then adding this to the top of my category.php file:

<?php
$sortbystate = $_GET['state'];

//get the current cat id
$cat_obj = $wp_query->get_queried_object();
$cat_id = $cat_obj->cat_ID;

$states = $wpdb->get_col("SELECT meta_value FROM $wpdb->postmeta
LEFT JOIN $wpdb->posts ON($wpdb->postmeta.post_id = $wpdb->posts.ID)
LEFT JOIN $wpdb->term_relationships ON($wpdb->posts.ID = $wpdb->term_relationships.object_id)
LEFT JOIN $wpdb->term_taxonomy ON($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
WHERE $wpdb->term_taxonomy.term_id = $cat_id
AND $wpdb->term_taxonomy.taxonomy = 'category'
AND $wpdb->postmeta.meta_key = 'state'
ORDER BY $wpdb->postmeta.meta_value ASC
");

?>

And for the links to click to actually filter the posts:

<ul class="track-filters">
<?php foreach ($surfaces as $surface) {
echo '<li><a href="?surface='.$surface.'">'.$surface.'</a></li>';}?>
</ul>

The problem is, when I have more than one track with the same state, the SQL query is outputting each instance of that custom field. That’s why there are 2 Alabamas right now.

Can anyone tell me how to modify my query so that it will only display one result for each state?

Thanks very much.